Printing on Conductive Substrate Material

ABSTRACT

An electrophotographic printing control apparatus serves for transferring an image via an electrically biasable ITM drum to a conductive substrate. The electrically biasable ITM drum is in contact with the conductive substrate over a contact period during which the image is transferred to the conductive substrate. The printing control apparatus includes a bias unit which provides electrical bias to the ITM drum and with short circuit protection, and a bias switching unit controllable to cut bias from the bias unit to the ITM drum during the contact period such that the ITM drum is unbiased when in contact with the conductive substrate.

RELATED APPLICATIONS

This patent application claims priority to PCT/US2005/039200, havingtitle “Printing on Conductive Substrate Material”, filed on 27 Oct.2005, commonly assigned herewith, and hereby incorporated by reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to printing on conductive substratematerial and, more particularly, but not exclusively to printing on websof materials such as aluminum using an electrophotographic printingmachine.

Electrophotographic printing machines generally use a two-transfersystem of printing in which an electrophotographic image is formed on afirst drum using a laser beam shone onto a photoelectric material. Anelectrostatic image is formed in the photoelectric material by the laserbeam and then ink is drawn into the electrostatic image. The image soformed is then transferred in a first transfer operation onto a blanketcarried by an intermediate transfer drum, known as the ITM drum. Asecond transfer operation occurs when the image is transferred from theblanket onto the printing substrate which is held on a third drum, knownas the impression drum.

Printing devices for separate sheets of paper are known that printcolors by carrying out separate transfer operations for each color. Thatis to say they rotate the printing substrate over the impression drumseveral times, each time transferring the image per one color. Whenprinting on web, multiple rotation of the drum for a single section ofprinting is not possible since the web is continuous. Therefore machinesfor printing on web use what are known as one-shot printing techniques,in which all of the printing images for all of the colors are gatheredon the ITM drum and then transferred in a single rotation onto the websubstrate.

Referring now to the drawings, FIG. 1 schematically illustrates a crosssectional view of an electrostatic printing assembly 1, according to theteaching of prior art. Apparatus 1 comprises an electrostatic drum 10arranged for rotation about an axle 12. Drum 10 is typically formed withan imaging surface 16, e.g., a photoconductive surface. Surface 16 istypically of a cylindrical shape.

A charging unit 18, which can be a corotron, a scorotron, a rollercharger or any other suitable charging unit known in the art, uniformlycharges surface 16, for example, with positive charge.

Continued rotation of the drum 10 brings surface 16 into image receivingrelationship with an exposing unit 20, which focuses one or morescanning laser beams onto surface 16 to scan a desired image. The laserbeams selectively discharge surface 16 in the areas struck by light,thereby forming an electrostatic latent image. Usually, the desiredimage is discharged by the light while the background areas are leftelectrostatically charged. Thus, the latent image normally includesimage areas at a first electrical potential and background areas atanother electrical potential. Unit 20 may be a modulated laser beamscanning device, an optical focusing device or any other imaging deviceknown in the art.

Continued rotation of the drum 10 brings imaging surface 16, now bearingthe electrostatic latent image, into a developing unit 22, whichtypically comprises electrodes 24 operative to apply a liquid toner orink on surface 16, so as to develop the electrostatic latent image. Theliquid toner can comprise charged solid particulates dispersed in acarrier liquid. The solid particulates are typically charged to the samepolarity of the photoconductor. Thus, due to electrostatic repulsionforces, ink particles adhere to areas on the photoconductorcorresponding to the image regions, substantially without adhering to(developing) the background regions. In this manner a developed image isformed on surface 16.

Following application of liquid toner thereto, surface 16 typicallypasses through other rollers (not shown) which ensure that the inksurface is appropriate for transfer to ITM drum 40. A first ink transferthen occurs, in which the liquid image is transferred, typically viaelectrostatic attraction, from drum 10 to ITM drum 40, rotating in theopposite direction of drum 10. In order for the first transfer to occur,an electrical bias is needed in the direction of image transfer. Thedrums are therefore generally biased negatively by a bias unit 44, sothat a forward bias leads from electrostatic drum 10 to ITM drum 40.

Subsequently, the image experiences a second transfer, typically aidedby heat and pressure, from ITM drum 40 to a substrate 42, which issupported by an impression drum 43.

Following the transfer of the liquid image to ITM drum 40, imagingsurface 16 is cleaned to remove ink traces. Residual charge left onsurface 16 can be removed, e.g., by flooding surface 16 with light froma lamp 58.

The electronic biasing provided by biasing unit 44 is problematic forprinting on a conductive web substrate. Biasing unit 44 typicallyutilizes a voltage source-type power supply with a high voltage rating.The power supply is designed to fail when a high current is drawn,bringing about collapse of the bias path and thus failure of printing.Generally such failure only occurs in the rare event of a short circuitwithin the printing machine, however a problem arises when the web beingprinted is conductive, for example in the case of printing on aluminumsheet, say in the form of foil. In such a case the conductive substratemust contact the drum for the ink transfer to succeed. However, at thetime the ink is being transferred to the substrate, ink is already beingtransferred to the ITM drum for the next operation, so as not to losecycles within the machine. Thus a short circuit is formed through theprinting substrate which is itself conductive, to earthed parts of theprinting machine, giving rise to current leakage which is generallysufficient to collapse the bias and therefore stop the printing.

In the past a solution was found to allow the printing of conductivewebs by isolating the conductive printing substrate from the rest of themachine. However such a solution is not practical in machines withsophisticated web feeding elements such as suction elements, since thesuction elements are themselves made of conducting material and have tocontact the web in order to work.

There is thus a widely recognized need for, and it would be highlyadvantageous to have, a means that would allow electrophotographicprinting of conductive web substrates without being liable to currentleakage.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan electrophotographic printing control apparatus serves fortransferring an image via an electrically biasable ITM drum to aconductive substrate. The electrically biasable ITM drum is in contactwith the conductive substrate over a contact period during which theimage is transferred to the conductive substrate. The printing controlapparatus includes a bias unit which provides electrical bias to the ITMdrum and with short circuit protection, and a bias switching unitcontrollable to cut bias from the bias unit to the ITM drum during thecontact period such that the ITM drum is unbiased when in contact withthe conductive substrate.

According to a second aspect of the present invention there is providedan electrophotographic printing control apparatus for transferring animage via an electrically biasable ITM drum to a conductive substrate.The electrically biasable ITM drum is in contact with the conductivesubstrate over a contact period during which the image is transferred tothe conductive substrate. The printing control apparatus includes a biasunit which provides electrical bias to the ITM drum and with shortcircuit protection, and a switching and control unit. The switching andcontrol unit cuts the bias from the bias unit to the ITM drum during thecontact period so that the ITM drum is unbiased when in contact with theconductive substrate. The switching and control unit also suspends afirst image transfer to the electrically biasable ITM drum during thecontact period by introducing a first and a second null cycles of theelectrostatic drum, such that the contact period extends from within thefirst null cycle to within the second null cycle.

According to a third aspect of the present invention there is providedan electrophotographic printing control apparatus for transferring animage via an electrically biasable ITM drum to a conductive substrate.The electrically biasable ITM drum is in contact with the conductivesubstrate over a contact period during which the image is transferred tothe conductive substrate. printing control apparatus includes a meansfor providing electrical bias to the ITM drum and a means for cuttingbias from the bias unit to the ITM drum during the contact period suchthat the ITM drum is unbiased when in contact with the conductivesubstrate.

According to a fourth aspect of the present invention there is provideda method of printing a conductive substrate using electrophotographicprinting. The method is performed by applying bias to a drum-based imagetransfer mechanism, under the bias carrying out a first image transferover the transfer mechanism, disconnecting the bias, and carrying out asecond image transfer from the drum-based image transfer mechanism tothe conductive web.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. The materials, methods, andexamples provided herein are illustrative only and not intended to belimiting.

Implementation of the method and system of the present inventioninvolves performing or completing certain selected tasks or stepsmanually, automatically, or a combination thereof. Moreover, accordingto actual instrumentation and equipment of preferred embodiments of themethod and system of the present invention, several selected steps couldbe implemented by hardware or by software on any operating system of anyfirmware or a combination thereof. For example, as hardware, selectedsteps of the invention could be implemented as a chip or a circuit. Assoftware, selected steps of the invention could be implemented as aplurality of software instructions being executed by a computer usingany suitable operating system. In any case, selected steps of the methodand system of the invention could be described as being performed by adata processor, such as a computing platform for executing a pluralityof instructions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin order to provide what is believed to be the most useful and readilyunderstood description of the principles and conceptual aspects of theinvention. In this regard, no attempt is made to show structural detailsof the invention in more detail than is necessary for a fundamentalunderstanding of the invention, the description taken with the drawingsmaking apparent to those skilled in the art how the several forms of theinvention may be embodied in practice.

In the drawings:

FIG. 1 schematically illustrates a cross sectional view of a prior-artelectrostatic printing apparatus.

FIG. 2 is a simplified block diagram of an electrophotographic printingcontrol apparatus according to an embodiment of the present invention.

FIG. 3 is a simplified timing diagram of ITM drum biasing, relative tothe first and second image transfer cycles, according to an embodimentof the present invention.

FIG. 4 is a simplified timing diagram of ITM biasing for a printingcycle which includes two null periods for a single color image,according to an embodiment of the present invention.

FIG. 5 is a simplified timing diagram of for a one-shot YMCK (yellow,magenta, cyan, and black) printer which applies two layers of white inkfollowed by the four colors, according to an embodiment of the presentinvention.

FIG. 6 is a simplified flowchart of a method of printing a conductivesubstrate using electrophotographic printing, according to an embodimentof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present embodiments comprise an apparatus and a method for printingon a conductive web substrate.

In the present embodiments, printing on a conductive substrate isperformed on an electrophotographic printer by removing the electricalbias to the ITM drum at all times that the ITM drum is in contact withthe conductive substrate, specifically during the second ink transfer.Thus no current surge is detected by the bias unit during image transferto the conductive substrate, and a shutdown of the bias unit isprevented. While the bias unit is disconnected, other operations thatneed bias on the ITM drum (such as the transfer of the image from theelectrostatic drum to the ITM drum) are avoided. In a furtherembodiment, such operations are suspended during periods in which theITM drum is unbiased by adding one or more null cycles (during which thedrums turn but no printing substrate is fed through) for each printedimage. As will be discussed below, the addition of null cycles reducesthe throughput of the printer, but enables printing on conductive websubstrates.

The principles and operation of an apparatus and method according to thepresent invention may be better understood with reference to thedrawings and accompanying description.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

Parts that are the same as those in previous figures are given the samereference numerals and are not described again except as necessary foran understanding of the present embodiment. In the following the term“first transfer” refers to image transfer from the electrostatic drum tothe ITM drum, and the term “second transfer” refers to image transferfrom the ITM drum to the substrate.

Reference is now made to FIG. 2 which is a simplified block diagram ofan electrophotographic printing control apparatus according to anembodiment of the present invention. Printing control apparatus 200provides biasing and control to a printer assembly 230 with anelectrically biasable ITM drum 40, and operating substantially asdescribed above. As discussed above, electrical biasing of ITM drum 40ensures the transfer of ink from electrostatic drum 10 to ITM drum 40.

Printing control apparatus 200 contains bias unit 210, which provideselectrical bias to ITM drum 40, and bias switching unit 220. Bias unit210 has short circuit protection 215 which shuts down electrical biasingwhen a current surge is detected. Bias switching unit 220 serves to cutoff the bias from bias unit 210 to the ITM drum during the contactperiod, so that ITM drum 40 is unbiased when in contact with theconductive substrate 42.

In an additional embodiment, printing control apparatus 200 furtherincludes cycle control unit 225, which switches printing assembly 230between first transfer operations and the second transfer operations.Cycle control unit 225 and bias switching unit 220 thus work in concertto time the transfer cycles and biasing cycles so that biasing isapplied only at the appropriate times in the image transfer process.

The cycle control unit 225 may be configured to ensure that the firsttransfer (from electrostatic drum 10 to ITM drum 40) and the secondtransfer (from ITM drum 40 to substrate 42) are offset temporally. Thusbias switching unit 220 can apply electrical biasing for the first inktransfer, and turn off electrical biasing for the second ink transferduring which ITM drum 40 is in contact with conductive substrate 42.Consequently, no conductive path is formed via conductive substrate 42,the short circuit protection 215 of bias unit 210 is not activated, andprinting on conductive substrate 42 may be accomplished. Printingcontrol apparatus 200 is appropriate for printing on a conductive websubstrate, for which the prior-art solution of isolating conductivesubstrate 42 from the rest of the printer is particularly difficult toimplement.

In a further embodiment, cycle control unit 225 is configured to providea first delay between removing the bias voltage and starting the secondtransfer. The delay enables decay of the bias voltage before ITM drum 40makes contact with substrate 42. Likewise, cycle control unit 225 mayprovide a delay at the end of the second transfer to enable the biasvoltage to rise to the required level before the next first transferstage is performed.

Reference is now made to FIG. 3 which is a simplified timing diagram ofITM biasing relative to the first and second transfer cycles, accordingto the present embodiment. In stage A, the first transfer is performedwith biasing voltage on, to transfer the image to the ITM drum.Following stage A, a delay period occurs to enable the decay of the biasvoltage prior to beginning the image transfer to the substrate in stageB. After stage B, a delay period occurs to enable the bias voltage torise to the required level. The next printing cycle then begins at stageC, with a new image transfer to the ITM drum. It is readily seen fromFIG. 3 that a biasing voltage is present for first transfer operations,but is not present for second transfer operations.

In a typical four-shot printer, a separate printing assembly (i.e.electrostatic drum, ITM drum and impression drum) is provided for eachcolor, and the different colors are applied consecutively to thesubstrate in order to form a color image. Commonly the second transferof each ink layer starts about half a cycle after the first transfer.This means that the second transfer of each layer starts at the middleof the first transfer of the same layer and ends during the firsttransfer of the next layer.

In one-shot printers all the ink layers are first accumulated on the ITMdrum by performing a series of first transfer operations. A singlesecond transfer operation then transfers all layers (i.e. colors) to thesubstrate. In one-shot printers, the second transfer typically starts atthe middle of the first transfer of the final layer of the currentimage, and ends during the first transfer of the next image. If theabovedescribed timing of the transfer cycles is maintained for printinga conductive web substrate, cutting the electrical bias to the ITM drumduring the second transfer may interfere with the first transfer of twoink layers.

In a further embodiment, null cycles are introduced into the printingcycle, during which first transfer operations are suspended. In thepresent embodiment, cycle control unit 225 suspends the first imagetransfer to ITM drum 40 during the contact period by introducing one ormore null cycles, desirably two, of the electrostatic drum. During thenull cycles, the drums spin but no web substrate is fed through printerassembly 230. The second transfer may then be performed from a mid-pointof the first null cycle until a mid-point of the second one, withoutinterfering with the first transfer process. In this case, all firsttransfers are performed with full bias and there is still ample time forbias decay and rise time. Contact between the ITM drum and the substrateis prevented during those times that a bias is applied.

Reference is now made to FIG. 4 which is a simplified timing diagram ofITM drum biasing for a print cycle which includes two null periods aftertransferring a single color to the ITM drum. During cycle 1 (stage A),the first transfer is performed with biasing voltage on, to transfer theimage to the ITM drum. Following stage A, two null cycles (cycles 2 and3) occur during which first image transfer operations are suspended. Thesecond image transfer in stage B is performed in the middle of the twonull cycles, after the biasing voltage has decayed. The biasing voltageis then reapplied, and stage C is performed during cycle four, after thetwo null cycles.

For one-shot printing of four-color images on a conductive substrate,two null cycles are added for each four cycles, and productivity isreduced by one third. In practice, the conductive substrates arealuminum and white ink is printed under the image in order to givenormal colors to the printed image. In this case, the image has six inklayers (YMCK and two white layers). and one null cycle is typicallyadded to enhance image drying. In this case only one more null cycle isadded for the present embodiment. This results in productivity reductionof approximately 14 percent.

FIG. 5 illustrates the timing for a one-shot YMCK (yellow, magenta,cyan, and black) printer which applies two layers of white ink followedby the four colors. During cycles 1-6, six consecutive first transfersare performed to apply the two white layers and the four colors to theITM drum. Cycles 7 and 8 are null cycles during which first transferoperations are not performed. During stage B, a single second transferoperation transfers the multi-color image to the substrate. Stage B isperformed in the middle of cycles 7 and 8, after the bias voltage hasdecayed. After the two null cycles, at the end of cycle 9, biasing isrestored and image transfer to the ITM drum resumes.

Reference is now made to FIG. 6, which is a simplified flowchart of amethod for printing a conductive substrate using electrophotographicprinting, according to an embodiment of the present invention. In step610 a bias is applied to a drum-based image transfer mechanism. Underthe bias, a first image transfer is carried out over the transfermechanism in step 620. In step 630 the bias is disconnected, desirablysubstantially at a midpoint of the first null cycle. Finally, a secondimage transfer from the drum-based image transfer mechanism to theconductive web is carried out in step 640.

The present method may also include the step of adding a first nullcycle of an image electrostatic source drum of the drum-based imagetransfer mechanism between the first transfer and the second transferand adding a second null cycle following the second image transfer andpreceding a first image transfer of a next image. When the conductivesubstrate is aluminum, requiring printing of white layers and a dryingnull cycle for ink drying, the drying null cycle may be used as one ofthe first and second null cycles.

It is expected that during the life of this patent many relevant devicesand systems will be developed and the scope of the terms herein,particularly of the terms electrophotographic printing, image transfer,biasing, and conductive substrate is intended to include all such newtechnologies a priori.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents, and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

1. Electrophotographic printing control apparatus for transferring animage via an electrically biasable intermediate transfer (ITM) drum to aconductive substrate, said electrically biasable ITM drum being incontact with said conductive substrate over a contact period duringwhich said image is transferred to said conductive substrate, theapparatus comprising: a bias unit with short circuit protection, saidbias unit for providing electrical bias to said ITM drum; and a biasswitching unit controllable to cut bias from said bias unit to said ITMdrum during said contact period such that said ITM drum is unbiased whenin contact with said conductive substrate.
 2. Apparatus according toclaim 1, further comprising a cycle control unit associated with saidbias switching unit, operable to suspend a first image transfer to saidelectrically biasable ITM drum during said contact period.
 3. Apparatusaccording to claim 2, further comprising an electrostatic drum fromwhich said first image transfer to said ITM drum is carried out, whereinsaid suspending image transfer to said electrically biased drumcomprises introducing a null cycle of said electrostatic drum. 4.Apparatus according to claim 3, wherein said cycle control unit isconfigured to introduce two of said null cycles per image printingoperation, such that said contact period extends from within said firstnull cycle to within said second null cycle.
 5. Apparatus according toclaim 4, wherein said contact period extends from substantially amid-point of said first null cycle to substantially a mid-point of saidsecond null cycle.
 6. Apparatus according to claim 5, wherein saidcontact period extends from a mid-point of said first null cycle to amid-point of said second null cycle.
 7. Apparatus according to claim 1,configured to carry out a first image transfer to said ITM drum and saidimage transfer to said substrate as a second transfer, wherein saidcycle control unit is configured such that said first transfer and saidsecond transfer are offset temporally.
 8. Apparatus according to claim7, wherein said cycle control unit is configured such that said bias isapplied during said first transfer and is not applied during said secondtransfer.
 9. Apparatus according to claim 8, wherein said cycle controlunit is configured to provide a first delay between removing said biasand starting said second transfer.
 10. Apparatus according to claim 9,wherein said first delay is set such as to allow bias decay before saidsecond transfer begins.
 11. Apparatus according to claim 9, wherein saidcycle control unit is configured to provide a second delay between anend of said second transfer and reapplying said bias.
 12. Apparatusaccording to claim 11, wherein said second delay is set such as to allowfor bias rise time.
 13. Apparatus according to claim 1, wherein saidsubstrate is a conductive web.
 14. Electrophotographic printing controlapparatus for transferring an image via an electrically biasable ITMdrum to a conductive substrate, said electrically biasable ITM drumbeing in contact with said conductive substrate over a contact periodduring which said image is transferred to said conductive substrate, theapparatus comprising: a bias unit with short circuit protection, saidbias unit for providing electrical bias to said ITM drum; and aswitching and control unit controllable to cut bias from said bias unitto said ITM drum during said contact period such that said ITM drum isunbiased when in contact with said conductive substrate, and to suspenda first image transfer to said electrically biasable ITM drum duringsaid contact period by introducing a first and a second null cycles ofsaid electrostatic drum, such that said contact period extends fromwithin said first null cycle to within said second null cycle. 15.Electrophotographic printing control apparatus for transferring an imagevia an electrically biasable ITM drum to a conductive substrate, saidelectrically biasable ITM drum being in contact with said conductivesubstrate over a contact period during which said image is transferredto said conductive substrate, the apparatus comprising: a means forproviding electrical bias to said ITM drum; and a means for cutting biasfrom said bias unit to said ITM drum during said contact period suchthat said ITM drum is unbiased when in contact with said conductivesubstrate.
 16. Method of printing a conductive substrate usingelectrophotographic printing, the method comprising: applying bias to adrum-based image transfer mechanism; under said bias carrying out afirst image transfer over said transfer mechanism; disconnecting saidbias; and carrying out a second image transfer from said drum-basedimage transfer mechanism to said conductive web.
 17. The method of claim16, comprising adding a first null cycle of an image electrostaticsource drum of said drum-based image transfer mechanism between saidfirst transfer and said second transfer and adding a second null cyclefollowing said second image transfer and preceding a first imagetransfer of a next image.
 18. The method of claim 17, wherein saidconductive substrate is aluminum, requiring printing of white layers anda drying null cycle for ink drying, the method comprising using saiddrying null cycle as one of said first and second null cycles.
 19. Themethod of claim 16, wherein said disconnecting said bias is carried outsubstantially at a midpoint of said first null cycle.